Shoot apical meristems are clusters of cells borne at the tips of branches that contain a small number of "stem cells" that act as the source of new cells for growth and development. The ARGONAUTE (AGO) family of proteins is involved in stem cell function in organisms as diverse as Drosophila and Arabidopsis.The Arabidopsis PINHEAD and ARGONAUTE genes are related genes whose activity is required for maintenance of the shoot apical meristem. Lowering of PNH/AGO activity results in the shoot apical meristem being replaced by a terminal leaf. Conversely, ectopic PINHEAD leads to extra growth. Members of the AGO gene family appear to be involved in a variety of RNA-mediated processes. Their specificity is thought to be mediated through a mechanism involving microRNAs (small RNAs about 22 bp in length). We have identified two genes involved in development at the shoot apex whose translation is likely regulated by PNH/AGO and will carry out experiments to understand the mechanism by which PNH/AGO regulate the translation of these genes. We will identify additional targets of PNH/AGO control by applying DNA chip analysis to polysome profiles. We will identify other components of the PNH/AGO genetic pathway through a suppressor/enhancer screen we have developed. We will also explore the possibility that dominant PHB mutations disrupt binding of a regulatory miRNA. Dominant PHB mutations cause increased and ectopic mRNA accumulation and decreased methylation (in cis) of the PHB gene. We will attempt to identify the AGO-like gene responsible for regulating PHB. Finally, we will perform experiments to determine to what degree PNH and AGO functions differ from one another and what gene or protein sequences are responsible for this specificity.